The Federal Aviation Administration thinks 8.8% fewer passengers will board major U.S. airlines this fiscal year, while 2.4% fewer international boardings will occur in 2009, thanks to the economic slowdown.

Due to the current worldwide economic downturn, the FAA’s 16-year forecast for 2009-2025 predicts domestic passenger enplanements to decrease by 7.8 percent in 2009 and then grow an average of 2.7 percent per year during the remaining 15-year forecast period. While last year the FAA predicted the U.S. airlines would reach a billion passengers a year by 2016, today’s forecast projects U.S. airlines to reach one billion air travelers annually by 2021. The number of passengers on U.S. airlines domestically and internationally is forecast to increase from 757.4 million in 2008 to 1.1 billion in 2025.

U.S. aircraft operations are predicted to experience a 5.7 percent decrease in 2009 from 2008 levels. Beginning in 2010, the agency expects operations to grow at an average annual rate of 1.5 percent for the remainder of the forecast period.

Of course, crystal balls over at the the Federal Aviation Administration are just as cloudy as they are everywhere else. We sought out some quick second opinions on the FAA numbers from airline industry analysts and experts. Here’s what they had to say:

Mo Garfinkle, GCW Consulting: I think the forecast is a bit optimistic. I would expect domestic passenger enplanements to decline slightly more than 9.0% this year, with next year starting slow and enplanement growth for the full year at about 2%. The impact of the recession on people’s pocketbooks is quite evident. What we haven’t seen is the psychological impact. I think it will take substantial time for people to feel financially comfortable enough again to return to something like their old ways of multiple holidays.

In the long-term, growth will be restored. That’s why the FAA needs to be prepare for it. In some ways, the current downturn is a blessing for the FAA. It takes the problems of overcapacity off the table for now and gives them the time to do what they should have done 20 years ago.

Tom Botts, Hudson Crossing: Those numbers feel highly optimistic to me unless you believe we are truly at the low point in the economy today — as in right now. I hope …